Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

Abstract

Abstract-Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (∼100-500 μm) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to ∼3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reversezoned, and also have ∼10-30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augitebearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 °C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.

Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle

“…Samples were kindly provided by the NASA meteorite working group (MWG), the National Table 1 Details of meteorite samples studied. Olivine compositions are from this study except D (Downes et al, 2008), SG (Singletary and Grove, 2003), G (Goodrich et al, 2006), and WR (Warren and Rubin, 2010 Our meteorites from Sahara have been less studied, and for most of them, only their descriptions in the Meteoritical Bulletin are available. They are all regular ureilites.…”

“…A large clast (ALM-A) has been recently discovered among the numerous stones of the Almahata Sitta fall, (Clayton and Mayeda, 1996) and δ 56 Fe in the bulk rocks vs. the composition of the olivine cores (data from Table 2, and mainly from Downes et al, 2008, Singletary and Grove, 2003, Goodrich et al, 2006and references therein). and demonstrate that at least part of these melts was trachyandesitic (Bischoff et al, 2014).…”

Petrogenesis of augite-bearing ureilites Hughes 009 and FRO 90054/93008 inferred from melt inclusions in olivine, augite and orthopyroxene

“…In this paper, we address the origin of the relatively rare augite-bearing ureilites (8 out of 248 samples), which appear to be cumulates rather than residues (Goodrich et al, 2004(Goodrich et al, , 2006. This interpretation is based largely on the Fe/Mg-Fe/Mn compositions of their olivine, which are displaced from the trend of the olivine-lpyx ureilites ( Fig.…”

“…They can be subdivided (Goodrich et al, 2004(Goodrich et al, , 2006 into three groups based on the identity of their pyroxenes: olivine-pigeonite, olivine-orthopyroxene and augite-bearing. Olivine-pigeonite ureilites (the most abundant) contain uninverted pigeonite (Wo $ 6-13) as their dominant pyroxene, and have Fo values ranging from $76 to 87 among samples.…”

Osmium isotope systematics of ureilites

“…On the basis of their silicate mineralogy, the monomict ureilites are classified into three different types: olivine-pigeonite, olivine-orthopyroxene, and augite-bearing. Succinct descriptions of the classification of monomict ureilites are given in Goodrich et al (2004Goodrich et al ( , 2006. Most monomict ureilites show coarse-grained igneous textures with mineral grains joining in abundant 120° triple junctions.…”